By definition, atoms of a given chemical element have the same atomic number. However, atoms of a chemical element may have different mass numbers. Specifically, different isotopes of a chemical element will have the same number of protons, but a different number of neutrons, in their respective nuclei. As one might expect, the separation of one isotope from another using chemical techniques is somewhat difficult because different isotopes of the same chemical element typically have almost identical chemical properties. On the other hand, techniques such as gravimetric and centrifugal separation processes that rely on a weight differential for separation are generally inefficient for isotope separation because of the relatively small weight differential that exists between isotopes of the same chemical element.
Particularly relevant for the present discussion is the fact that a plasma can be created from a material that contains isotopes of a chemical element. In greater detail, a plasma is a high-temperature, highly ionized, gaseous discharge that typically includes electrons, ions, and electrically neutral particles. Although the various particles in a plasma may be positively or negatively charged, the relative number of negative charges (e.g. electrons and negative ions) and positive ions that exist in a plasma are such that the plasma, as a whole, is electrically neutral. Nonetheless, a plasma is typically so highly ionized that it is electrically conductive and can be influenced by electric and magnetic fields.
One separation technique that has been previously suggested takes advantage of the fact that the orbital motions of charged particles (e.g. ions) in a plasma under the influence of crossed electric and magnetic fields will differ from each other according to their respective mass to charge ratios. For example, U.S. Pat. No. 6,096,220, which issued on Aug. 1, 2000 to Ohkawa, for an invention entitled “Plasma Mass Filter” and which is assigned to the same assignee as the present invention, discloses a device which relies on the different, predictable, orbital motions of charged particles in crossed electric and magnetic fields to separate charged particles from each other according to their respective mass to charge ratios. U.S. Pat. No. 6,096,220 is hereby incorporated by reference.
In the filter disclosed in Ohkawa '220, a cylindrical shaped wall surrounds a chamber and defines a longitudinal axis for the filter. Coils are provided to establish an axially oriented magnetic field throughout the chamber. The filter also includes electrodes for generating an electric field in the chamber that is oriented substantially radially and outwardly from the axis (e.g. a parabolic electric field having a positive potential on the axis relative to the wall which is typically at a zero potential). With this cooperation of structure, both the magnetic and the electric fields are substantially uniform both azimuthally and axially.
As further disclosed in Ohkawa '220, this configuration of applied electric and magnetic fields causes ions having relatively low mass to charge ratios (hereinafter referred to as low mass ions) to be confined inside the chamber during their transit of the chamber. On the other hand, ions having relatively high mass to charge ratios (hereinafter referred to as high mass ions) are not so confined. Instead, these high mass ions are ejected from the plasma and into the wall of the chamber (or a collector positioned near the wall) before completing their transit through the chamber. The demarcation between high mass particles and low mass particles is a cut-off mass, MC, which is established by setting the magnitude of the magnetic field strength, Bz, the positive voltage along the longitudinal axis, Vctr, and the distance from the axis to the wall, “awall”. The cut-off mass, MC, can then be determined with the expression:MC=zeawall2(Bz)2/8Vctr where “ze” is the ion charge.
The Ohkawa '220 patent further discloses an operating procedure for the plasma mass filter in which a plasma throughput, Γ, is established such that the plasma density remains below a defined collisional density, nc. More specifically, as used herein, the “collisional density,” nc, is defined as being a plasma density wherein there is a probability of “one” that an ion collision will occur within a single orbital rotation of an ion around a rotation axis parallel to the chamber axis under the influence of crossed electric and magnetic fields (Er×Bz). In other words, a collisional density, nc, is established when it is just as likely that an ion will collide with another ion, as it is that the ion will not collide with another ion during a single orbital rotation.
In order to improve the plasma throughput, Γ, of a plasma filter, however, it may be desirable to operate the filter with plasma densities above the collisional density, nc. Along these lines, a system for separating ions by mass that is operable at plasma densities above the collisional density, nc, is disclosed and claimed in co-pending U.S. patent application Ser. No. 10/222,475 entitled “High Throughput Plasma Mass Filter” filed on Aug. 16, 2002 by Tihiro Ohkawa, and which is assigned to the same assignee as the present invention. U.S. patent application Ser. No. 10/222,475 is hereby incorporated by reference. For such a system that operates above the collisional density, nc, the separation factor, F, can be estimated using the relationship:F=ω2r2ΔM/2k0T where ω is angular speed, r is radius, k0 is Boltzmann constant, T is temperature (Kelvin) and ΔM is the difference in mass between the low mass ions and the high mass ions. Thus, for operation above the collisional density, nc, good separation factors can be obtained when the ion thermal energy is much less that the ion rotational energy.
In the absence of any specific provisions to control ion energy, ions are typically born during plasma initiation with an initial ion temperature that is relatively high (e.g. 500–700 eV). The downside of this relatively high initial ion energy is two-fold. First, as indicated above, the high initial ion energy adversely affects the separation factor for systems operating above the collisional density, nc. This is particularly concerning when the ions to be separated have a small mass differential, ΔM, for example when the ions to be separated are isotopes of the same chemical element, since the separation factor, F, is proportional to the mass differential, ΔM.
Another drawback associated with relatively high energy ions in a plasma is the heat that is transferred to the ion collector. In greater detail, when an ion strikes a collector, the full rotational energy of the ion is transferred to the collector in the form of heat. In general, collectors are only operational over a limited temperature range, and accordingly, a collector can only accommodate a limited amount of heat. In some cases, this restriction on collector heating can limit the operational throughput of the filter (i.e. throughput is limited to levels in which collector overheating does not occur). It follows that reduced energy ions can be used at higher throughputs, without collector overheat.
For the arrangement of fields described above, cold ions can be formed using a controlled plasma source that initiates a plasma within a relatively small radius of the longitudinal axis. Specifically, consider an ion rotating in the uniform field region. The orbit is circular and vr=0. The force balance dictates:ω=[½][−Ω±{Ω2−4αΩ}1/2]where Ω=eB0/M is the cyclotron frequency. The canonical angular momentum is given by:ρθ=±[eBzr2/2][1−4α/Ω]1/2.The ions with its mass equal to the cut-off mass have zero canonical momentum. The potential energy U of the Hamiltonian in the uniform field region is:U=e2Bzr2/8M−αeBzr2/2.For ions with the cut-off mass, MC, the potential becomes:U=0
When ions are produced by the ionization of neutral atoms, they have negligible kinetic energy. If they are produced at the location with U=0 and Ψ=0, the ions can form a cold rotating ring, provided that two regions are connected by the orbits. For example, the ions may be produced on the axis.
In light of the above, it is an object of the present invention to provide separation devices for converting a multi-constituent material into a plasma and thereafter separating particles in the plasma according to their respective mass to charge ratios. It is another object of the present invention to provide a mass separator that is operable at both collisional and collision-less plasma densities. It is still another object of the present invention to provide a mass separator that can be used to efficiently separate particles having a relatively small mass differential (e.g. isotopes of the same chemical element). It is yet another object of the present invention to provide a device for separating ions which can be used at relatively high throughputs without overheating the ion collectors. Yet another object of the present invention is to provide a separation device which is easy to use, relatively simple to implement, and comparatively cost effective.